This invention relates to a novel method for determining the stress at the convex surface of an optically-transparent body and particularly, although not exclusively, to a method of determining the stress at the external surface of the viewing window of a cathode-ray tube. The phrase "at the surface" is used herein to refer to a thin portion of a body at and near its surface as opposed to being in the interior or bulk of the body.
The susceptibility of the glass viewing window of a cathode-ray tube (CRT), or other optically-transparent body, to mechanical failure is determined largely by the magnitude and distribution of stresses induced in it during its manufacture. These stresses may be at the surface or in the bulk of the body. Stress at the surface is especially relevant because mechanical failures, such as a fracture in the body or an implosion of the viewing window of a CRT, generally originate at the free surfaces of the body and are governed by the stresses there.
The novel method employs the known elasto-optic effect whereby a medium becomes optically birefringent when it is stressed. The birefringence is measured by interference between differently polarized components of light. The determination of stress in the bulk of a body by such techniques is well known, as exemplified in U.S. Pat. Nos. 2,976,764 to W. L. Hyde et al, 3,183,763 to C. J. Koester and 3,902,805 to S. Redner. In general, stress as measured in the bulk, does not provide adequate information about the surface stresses. For example, the thermal effects employed during its manufacture will cause the surfaces of a fairly flat, free-standing glass body to be in compression, and the interior to be in tension. The compression and tension very nearly cancel the net elasto-optic effect observed along any light path that passes through the thickness of the body. As another example, the viewing window of a CRT, mechanically loaded by the air pressure on the outer side and vacuum on the inner side, has a nonzero resultant for the elasto-optic effect when measured along a light path that passes through its thickness. However, the outer side is in compression, the inner side is in tension, and the net observed effect is due to an unbalance of the oppositely-sensed stresses.
A number of methods have been used for determining the surface stress in a body. Each prior method is deficient in some respect when compared with the novel method. Strain gauges, elasto-optically sensitive coatings, and brittle coatings can measure only the changes in stress that result from forces applied after the body has been fabricated with stresses therein. Failure of the body is governed, however, by the absolute stress in the body, and not by the last changes in stress produced in the body. In differential refractometry, stress is determined by the elasto-optic effect whereby the critical angle for total internal reflection changes when the body is stressed. Because it is a noninterferometric technique, its sensitivity and accuracy are limited. This method appears to be useful primarily where large, compressive stresses are deliberately induced in the surfaces of materials in order to strengthen them. In the notch polariscope method, parallel notches are cut into the surface of the body to be measured, and light is directed through the body from one notch to the other. Detection and measurement of the birefringence are by conventional means, involving crossed polarizers and possibly compensators, etc. Measurements can be made as a function of depth below the surface. Drawbacks of this method are that it is destructive, and that cutting the notches may perturb the stress distribution in the body.
It has also been suggested privately that, if the surface of the body to be tested is somewhat convex, as is the case for a kinescope face panel, then a light beam can be propagated along a chord that travels near the surface and intersects the surface at input and output regions. This method is impractical because, when the effects of diffraction are taken into account, it is necessary that the beam have appreciable width in order for it not to diverge by more than its width as it propagates through the medium to be tested. This in turn implies that there is a minimum depth through which the lowest part of the beam traverses. For desired values of the propagation length, this depth may be undesirably far away from the surface. This method would be satisfactory if the light beam could be made to propagate only at the surface without spreading due to diffraction. In optical waveguides, light behaves exactly in this desired manner. However, in optical waveguides the light is usually confined to a thin film that has a refractive index that is higher than that of the substrate on which it is supported; as disclosed, for example, in U.S. Pat. Nos. 3,584,230 to P. K. Tien and 3,883,221 to W. W. Rigrod.
What makes the novel method possible is the phenomenon, reported by S. Sheem et al in Wave Electronics 1 (1974) 105-116 and S. Sheem in Applied Optics 14 (1975) 1854-1859, that a convex body whose refractive index is spatially homogeneous behaves like a planar object with a refractive index that increases towards the surface. Guided wave propagation can therefore occur solely along a single surface of such a body. In the latter article, there is described a method in which a light beam from a He-Ne laser is coupled through a prism to the surface of a cylindrical rod or to a cylindrical tube, and the light beam propagates along the convex surface of the body in a guided manner.